Drop scp transport

[agis]

It is hard to setup, not going to be used in production environments and overall rsync is a better solution. The only think we decided to add scp was for use in tests and as a more easy-to-setup default than rsync. However, it seems that it's not so easy since it still requires setting up SSH keys and also required more configuration to work as we want it (as opposed to rsync).

If scp doesn't provide the benefits we chose it for, we should drop it completely. For this, #52 should be done first so that we have an alternative (ie. running tests with rsync without having to setup an rsync daemon).

[agis]

scp proved useful for testing and development scenarios, so I believe we should leave it as is. The cost of maintaining it is close to 0 anyway.